The invention relates to fluidized bed apparatus and has particular application to fluidized bed combustion apparatus in steam generation apparatus. While the present invention has primary application to a combustion process in a steam generating system, it will be understood that the present invention may also be used in a wide variety of fluidized bed apparatus. Those skilled in the art will further recognize that fluidized beds have been used for decades in non-combustion reactions in which the thorough mixing and intimate contact of the reactants in a fluidized bed result in high product yield with improved economy of time and energy.
Fluidized bed combustion apparatus can burn coal efficiently at temperatures low enough to avoid many of the problems of combustion in other modes. The term "fluidized bed" refers to the condition in which solid materials are given free flowing, fluid-like behavior. As a gas is passed upward through a bed of solid particles, the flow of gas produces forces which tend to separate the particles from one another. At low gas flows, the particles remain in contact with other solids and tend to resist movement. This condition is referred to as a fixed bed. As the gas flow is increased, a point is reached at which the forces on the particles are just sufficient to cause separation. The bed is then deemed to be fluidized. The gas cushion between the solids allows the particles to move freely, giving the bed a liquid-like characteristic.
Fluidized bed combustion makes possible the burning of fuels having such a high concentration of ash, sulfur, and nitrogen that they would ordinarily be deemed unsuitable. By the use of this process it is possible, at least in some cases, to avoid the need for gas scrubbers while still meeting emissions requirements. In fluidized bed combustion, the fuel is burned in a bed of hot incombustible particles suspended by an upward flow of fluidizing gas. Typically the fuel is a solid such as coal, although liquid and gaseous fuels can be readily used. The fluidizing gas is generally combustion air and the gaseous products of combustion. Where sulphur capture is not required, the fuel ash may be supplemented by inert materials such as sand or alumina to maintain the bed. In applications where sulphur capture is required, limestone is used as the sorbent and forms a portion of the bed. There are two main types of fluidized bed combustion systems. They are referred to as bubbling fluid bed (BFB) types and circulating fluid bed (CFB) types. In the former air in excess of that required to fluidize the bed passes through the bed in the form of bubbles. The bubbling fluid bed is further characterized by modest bed solids mixing rate and relatively low solids entrainment in the flue gas. The circulating fluid bed (CFB) type is characterized by higher velocities and finer particle sizes. In" such systems the fluid bed surface becomes diffuse as solids entrainment increases, such that there is no longer a defined bed surface. Circulating fluid bed systems have a high rate of material circulating from the combustor to the particle recycle system and back to the combustor. The present invention has particular application to circulating fluid bed boilers although those skilled in the art may recognize other applications. Characteristics of apparatus of this general type are further described in the publication Combustion Fossil Power, edited by Joseph G. Singer, P. E. and published by Combustion Engineering, Inc.; a subsidiary of Asea Brown Boveri, 1000 Prospect Hill Road, Windsor, Conn. 06095, 1991.
In a conventional circulating fluidized-bed steam generator crushed fuel and sorbent are fed mechanically or pneumatically to the lower portion of a combustor. Primary air is supplied to the bottom of the combustor through an air distributor, with secondary air fed through air ports at one or more elevations in the lower part of the combustor. Combustion takes place throughout the combustor, which is filled with bed material. Flue gases and entertained solids leave the combustor and enter one or more cyclones where the solids are separated and fall to a seal pot. From the seal pot, the solids are recycled to the combustor. Optionally, some solids may be diverted through a plug valve to an external fluidized-bed heat exchanger (FBHE) and back to the combustor. In the FBHE, tube bundles absorb heat from the fluidized solids.
The air distribution within the fluidized bed is achieved with in the prior art apparatus by a plurality of mutually parallel stainless steel pipes that extend vertically from a base plate for the fluidized-bed. The upper end of each of these pipes is capped. Each pipe has a plurality of radially extending holes to direct air throughout the bed to achieve the fluidization. Each of the holes in these pipes are typically between 0.070 to 0.090 inches in diameter. Such pipes are usually cast and then the holes are drilled in the walls thereof. Because of the stainless steel material attempts to cast the pipes with removable pins to form the polls have been unsuccessful. Thus, the expense of manufacture of such pipes is substantial.
These prior art constructions are also vulnerable to plugging of the holes due to contaminants in the air being supplied to the fluidized-bed. This requires maintenance to clean out the holes. In some cases this may result in down time for the steam generation apparatus. Even if the steam generating apparatus is down for other maintenance there is always competition to perform various maintenance procedures within the furnace and it is therefore highly desirable to minimize the requirement for cleaning of the air distribution system and the fluidized-bed apparatus.
It is an object of the present invention to insure thorough mixing and intimate contact of the reactants in a fluidized bed apparatus.
It is an object of the invention to provide air distribution apparatus in a fluidized bed assembly that will require less maintenance.
Another object of the invention is to provide apparatus which may be manufactured more easily and at less cost than the known apparatus.
Still another object of the invention is provided apparatus which will more effectively distribute the air in the fluidized-bed apparatus and thus will not require as many individual structures in the prior art apparatus.